The invention relates to a return mechanism for an electrical limit switch, and more particularly to a return mechanism which enables low torque switch actuation and provides adequate return force for the actuator of the electrical limit switch.
Electrical limit switches are widely utilized for precise control and position indication in applications with machine tools, conveyors, transfer machines, and other types of modern high-speed production equipment. These devices are characterized by an enclosed body housing electrical switch contacts which are actuated by a movement imparted to an operating shaft protruding from the body. The operating shaft may be rotatable about its axis, or, alternatively, translatable along its axis and generally includes a biasing arrangement for returning the shaft to a neutral condition.
A particular species of limit switch constructions is that of the oil tight design, so called because this switch normally includes some type of seal for preventing oil, greases and related contaminants from entering into the area of the electrical switch contacts. In the oil tight limit switch, there is a main switch body sheltering the switch contacts upon which is mounted a smaller enclosure known as an operating head. An operating shaft is journaled within the head, inside of which there is means for converting shaft displacement into actuation of the associated switch contacts. In sequential control applications, it is often preferable to effect a momentary operation of these switch contacts followed by an automatic resetting of the limit switch. For this reason, it becomes necessary to provide a spring return mechanism within the operating head in order to force the operating shaft to a zero or neutral position.
Prior limit switches reflective of this general structure and behavior are disclosed in U.S. Pat. No. 3,373,257 issued to L. H. Matthias et al. on Mar. 12, 1968, and U.S. Pat. No. 3,749,860 issued to R. G. Crepeau on July 31, 1973. In these devices, an operating shaft protruding from an operating head is rotated to impart a vertical reciprocating motion to a plunger member which causes actuation of switch contacts located in a switch body below the head. A single compression spring acts upon a seat formed in the rotary operating shaft and serves to urge the shaft to a normal position upon its rotary displacement from such position.
Another related limit switch design is shown in U.S. Pat. No. 3,546,954 to M. D. Ustin. In this construction, twin compression springs of equal force are arranged in parallel to urge an operating shaft to a neutral position after being rotated to operate switch contacts in a body. This arrangement is similar to Matthias and Crepeau in the concept of utilizing some type of spring force mainly for resetting a limit switch.
While such return mechanisms are generally satisfactory in restoring the initial position of a rotary operating shaft of a limit switch, they often require an undesirably high operating torque for actuating the associated switch contacts. This is especially detrimental to the overall behavior of a machine in many light control applications. For example, a limit switch is often used as a pilot device to regulate conveyor operations, in which application an external lever mounted on the operating shaft of the switch is subjected to a wide range of stimuli imparted by moving components on the conveyor. When a conveyor component contacts the external lever to rotate the operating shaft, this mechanical movement opens or closes electrical switch contacts housed within the switch so as to stop the movement of the conveyor. Once physical engagement of the component against the external lever has been removed, the operating shaft will return to a neutral position under the influence of a spring return mechanism, and the limit switch will be reset for continued conveyor monitoring. In many cases, small loads being transported upon a conveyor are insufficient to overcome the biasing force supplied by the spring return mechanism. As a result, these small loads do not actuate the switch and are instead deflected away from the external lever of the limit switch. The small loads are many times pushed off the conveyor and damaged beyond repair.
Some switch constructions employed in the art have been designated to provide low torque operation but only when coupled with a minimum of return force. An example of such a switch is disclosed in U.S. Pat. No. 3,317,687 to W. F. Dehn issued May 2, 1967. In this device, a relatively light spring force bears against a flat surface of an operating camshaft urging the camshaft to a zero position. Upon application of an external force, the light spring allows for a low force rotation of the camshaft such that switch actuation may easily occur. With the release of the external force, the light spring weakly returns the camshaft to its initial position but has insufficient force to reset the switch without manual assistance.
Heretofore, prior art designs have incorporated equalized biasing mechanisms chiefly for purposes of restoring rotary actuators and switch mechanisms to a reset position, or enabling low torque operation of rotary actuators while sacrificing a loss in return force. The present invention is intended to improve upon these prior operating characteristics.